Drag effect of freewheeling fixed prop

[Sailfree]

Thank you all very much. I hadn't realised feathering props needed to be locked (which I can't do because Yanmar say it will invalidate warranty, so must be doing some damage). Also useful info on Autoprop.

You are incorrect you can lock a feathering or folding prop in astern to feather/fold the blades with a Yanmar gearbox as its only briefly and does not put the strain on the gearbox that a fixed prop does.

On many modern gearboxes locking a fixed prop astern causes strain inside the gearbox and on my Yanmar 75HP it locked the gearbox and I had to then start the engine in astern then quickly select neutral.

The sound of a wirling prop when sailing and wear on components offended me so I changed to a Variprop feathering prop. When sailing I select astern which encourages the blades to quickly go into the feathered position then go back to neutral (no wirling noise - wonderful). Unlike a fixed prop this is obviously not putting any strain on the gearbox as the selector does not lock in the astern position and engine/gearbox has now done 2600hrs without any problems.

 

[Sailfree]

I made the change for sailing performance but have been very pleased with motoring performance as well - particularly into any sort of chop.

Lets be clear a fixed prop with shaped blades correctly sized gives the best motoring performance going forwards.

A feathering prop has flat blades that align with the waterflow when sailing to cause least resistance. With flat blades v shaped blades there is an unexpected advantage in astern power as the feathering props flat blades are more efficient that the fixed shaped blades going the wrong way!! Great for a crash stop if misjudging a berthing or getting you off the putty. With charter skippers of various abilities that why I favour a feathering propellor.

If foward performance with a feathering prop is better then it may be because you have a better size for your boat. On my 75HP I changed from a 3 blade fixed to a 4 blade feathering. The reduction in diameter (greater gap between prop end and hull underside) results in good if not very similar forward performance but a 4 bladed fixed prop should bne superior.

I suspect many sailors with their new toys believe it to be better performance but unless the original fixed prop was the wrong size they are misleading themselves.

 

[cmedsailor]

A significant (to me) improvement - at least .25 of a knot, probably nearer .4.

At what sort of speeds do you see those improvements? Slow speeds or near hull speed?

 

[AntarcticPilot]

Lets be clear a fixed prop with shaped blades correctly sized gives the best motoring performance going forwards.

A feathering prop has flat blades that align with the waterflow when sailing to cause least resistance. With flat blades v shaped blades there is an unexpected advantage in astern power as the feathering props flat blades are more efficient that the fixed shaped blades going the wrong way!! Great for a crash stop if misjudging a berthing or getting you off the putty. With charter skippers of various abilities that why I favour a feathering propellor.

I don't know what make my feathering prop is (it was fitted when I bought her, and I can't see a maker's name on it), but the blades aren't flat. There may be some compromises to make them nest together nicely into a streamlined "tear drop" shape when folded, but the blades definitely aren't flat, and when open the prop looks very similar in shape to a fixed 2-blade prop.

 

[lw395]

I don't know what make my feathering prop is (it was fitted when I bought her, and I can't see a maker's name on it), but the blades aren't flat. There may be some compromises to make them nest together nicely into a streamlined "tear drop" shape when folded, but the blades definitely aren't flat, and when open the prop looks very similar in shape to a fixed 2-blade prop.

So it's a folding prop, not a feathering prop then?

Folding props seem to compromise the shape a bit to get more mass towards the end of the blades, so they centrifuge open in reverse.
I'd agree a fixed prop has less compromises, it's probably the best option for efficiency if the pitch and size are right.
But we are talking about sailing boats are we not?

a feathering prop where you can adjust max pitch might be a better compromise than a fixed or folding prop with not quite optimum pitch.

 

[AntarcticPilot]

So it's a folding prop, not a feathering prop then?

Folding props seem to compromise the shape a bit to get more mass towards the end of the blades, so they centrifuge open in reverse.
I'd agree a fixed prop has less compromises, it's probably the best option for efficiency if the pitch and size are right.
But we are talking about sailing boats are we not?

a feathering prop where you can adjust max pitch might be a better compromise than a fixed or folding prop with not quite optimum pitch.

Yes, it's a folder - mea culpa!

 

[Tranona]

a feathering prop where you can adjust max pitch might be a better compromise than a fixed or folding prop with not quite optimum pitch.

Most (but not all) feathering props are fixed pitch once they are set up. However some are adjustable, usually through adjustable stops, but have to be dismantled to carry out the adjustment and then the pitch is fixed. One advantage of this si that you can get different pitch in forward and reverse.

 

[NormanS]

I suspect that if you apply moderate braking force to the shaft, it will pitch itself to provide more torque until the point that it stalls and turns to feathered position. I've always been interested to experiment with a Brunton to see how well it would perform on a hybrid regenerative system. It would be good if it performed as I suspect and so you could let it freewheel to generate electricity, or brake the shaft to feather it for best sailing performance.

A few years ago when I had a fixed prop I tinkered about with a bike computer on my propshaft to count rpm and a variable torque brake (cord around an inline pulley, attached to a spring balance) to get some power curves from my freewheeling prop. It would be interesting to do the same with a Brunton. My folding prop does also spin if I leave it to freewheel and one day I might see how it performs too.

Bruntons say that you have to lock the prop.
Not long after I fitted an Autoprop, I was sailing over to Barra, in a good hash of wind, when suddenly something happened to the rudder. I could turn one way, but only with a lot of force, could I turn the other. It felt as if the leading edge of the rudder had split open, forcing it round. When I looked down below, I found that the prop shaft had started to turn. Locked shaft in gear -- problem solved. Steep learning curve.
P.S. The rudder has a balance strip, and I assume that deflected water from the rotating prop was hitting the balance strip. It now happens occasionally, but at least now I know what it is.

 

[Ric]

Bruntons say that you have to lock the prop.
Not long after I fitted an Autoprop, I was sailing over to Barra, in a good hash of wind, when suddenly something happened to the rudder. I could turn one way, but only with a lot of force, could I turn the other. It felt as if the leading edge of the rudder had split open, forcing it round. When I looked down below, I found that the prop shaft had started to turn. Locked shaft in gear -- problem solved. Steep learning curve.
P.S. The rudder has a balance strip, and I assume that deflected water from the rotating prop was hitting the balance strip. It now happens occasionally, but at least now I know what it is.

They say you have to lock the prop so that it stalls and feathers. There is no reason why you shouldn't let it turn as an impeller and use the torque to do something useful - e.g. power a generator.

If you google a lot, you can find examples of very fast catamarans with Brunton props which have had their engines inadvertently started by the torque generated by even a locked and feathered Brunton. Clearly they have some potential as impellers.

 

[charles_reed]

A feathering prop will almost certainly offer lower drag than a freewheeling fixed type but don't expect to see massive differences in sailing performance with one. I have had a Brunton's for years and it is excellent, but I cannot say I have noticed much difference when under sail.

As Charles points out, a feathering prop will continue to rotate unless the shaft is locked. With a Yanmar gearbox on shaft drive you need to select reverse to lock it. I cannot say whether the same applies to a saildrive.

In practice, over two Yanmar engines and 21 years of use I always lock my Autoprop in forward gear.#

I suspect forward or reverse it makes little difference.

A E Barrus will tell you that everything (especially use) invalidates the "warranty" - they just want to evade their responsibilities.

 

[charles_reed]

They say you have to lock the prop so that it stalls and feathers. There is no reason why you shouldn't let it turn as an impeller and use the torque to do something useful - e.g. power a generator.

If you google a lot, you can find examples of very fast catamarans with Brunton props which have had their engines inadvertently started by the torque generated by even a locked and feathered Brunton. Clearly they have some potential as impellers.

Only if locked in reverse - the comment is highly apocryphal - on a level with the moon being of green cheese.

 

[charles_reed]

Lets be clear a fixed prop with shaped blades correctly sized gives the best motoring performance going forwards.

A feathering prop has flat blades that align with the waterflow when sailing to cause least resistance. With flat blades v shaped blades there is an unexpected advantage in astern power as the feathering props flat blades are more efficient that the fixed shaped blades going the wrong way!! Great for a crash stop if misjudging a berthing or getting you off the putty. With charter skippers of various abilities that why I favour a feathering propellor.

If foward performance with a feathering prop is better then it may be because you have a better size for your boat. On my 75HP I changed from a 3 blade fixed to a 4 blade feathering. The reduction in diameter (greater gap between prop end and hull underside) results in good if not very similar forward performance but a 4 bladed fixed prop should bne superior.

I suspect many sailors with their new toys believe it to be better performance but unless the original fixed prop was the wrong size they are misleading themselves.

If I might amplify your statement - a correctly shaped and pitched fixed prop AND a fully self-pitching prop.

In fact a fixed prop is only fully effective at certain engine revs and within very limited load factors. It might be better than a folding propeller in forward gear, but the only time a folding prop is frequently not as good as a fixed is in reverse and both are definitely inferior to self-pitching and feathering props in reverse.

 

[Ric]

Only if locked in reverse - the comment is highly apocryphal - on a level with the moon being of green cheese.

No, Bruntons confirmed to me that this was possible and had happened. If the shaft is locked in reverse, there is an unfeathering moment on the blades (which also incidentelly gives some drag). If the waterspeed is high enough, this torque can eventually turn the engine and start it.

That is why I think a Brunton has a lot of potential as an impeller.

 

[NormanS]

They say you have to lock the prop so that it stalls and feathers. There is no reason why you shouldn't let it turn as an impeller and use the torque to do something useful - e.g. power a generator.

If you google a lot, you can find examples of very fast catamarans with Brunton props which have had their engines inadvertently started by the torque generated by even a locked and feathered Brunton. Clearly they have some potential as impellers.

I have explained at some length what happens with mine if I let it turn.
On a previous boat of mine which had a big, slow running Gardner engine with max revs of 900, and with a 3 : 1 reduction, turning a four bladed prop of 49" dia, it was quite possible to start the engine, using the propeller. The gearbox had to be in "ahead" of course.

 

[Ric]

I have explained at some length what happens with mine if I let it turn.

That's a function of the rudder arrangement of your boat, not a function of the prop itself which is what is being discussed here.

 

[bluemoongaffer]

Lets be clear a fixed prop with shaped blades correctly sized gives the best motoring performance going forwards.

I suspect many sailors with their new toys believe it to be better performance but unless the original fixed prop was the wrong size they are misleading themselves.

I replaced a fixed two blade with a feathering 3 blade and went up from 13in to 14in. Regardless of blade shape, I think this gives better performance

 

[bluemoongaffer]

At what sort of speeds do you see those improvements? Slow speeds or near hull speed?

Main benefit is from slow speeds up to just over 5 knots (hull speed on my boat is 6 knots.) Above this, it's mainly about angle to the wind, wind speed and sea state.

 

[cmedsailor]

I replaced a fixed two blade with a feathering 3 blade and went up from 13in to 14in. Regardless of blade shape, I think this gives better performance

Motoring performance? Of course it's better. Not only you have an extra blade but a larger prop too. So either the old two blader was very small (under-prop) or you are now over-prop.

 

[rob2]

There have been so many discussions on the subject and the research quoted as absolute proof. Unfortunately, the prop tested was a three bladed motor boat type prop which is not the usual first choice for a sailing boat. There are a number of variables in prop design which may prove or disprove the conclusions regarding the quoted research, particularly if combined with the variables of hull design. My own suspicion is that the differences observed will change substantially with changes in the area of the blades. A narrow two bladed sailing prop may well offer less drag if locked than if spinning when the slip magifies its cross sectional area - and if it can be stopped in line with a keel or deadwood it could prove almost invisible. Data based on a variety of blade designs would at least allow extrapolation within first order of magnitude.

Yes, my Flexofold likes to be stopped in gear to fold it.

Rob.

 

[srah1953]

Yes, my Flexofold likes to be stopped in gear to fold it.

Rob.

Yanmar requires that the engine be stopped before being put into gear to get blade to fold or fearther. Quote below from Yanmar Technical notes highlighted earlier-
"When sailing under sail with engine operation stopped, put control lever into Reverse, this to operate the folding/feathering prop to fold or feather. After this manoeuvre, put control lever back into Neutral.”